Key Points
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The initial promise for the development of cryoablation is to minimize the energy-dependent complications of atrial fibrillation (AF) ablation.
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Cryoablation reduces tissue fibrosis and modifies the healing process to minimize the potential of pulmonary vein (PV) stenosis and to eliminate the risk for atrioesophageal fistulas.
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Cryoablation is associated with lower risk for endocardial disruption/perforation to avoid tamponade, and decreased thrombosis formation to reduce the likelihood of stroke and transient ischemic attack.
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Cryoablation does not cause pain, which potentially reduces the need for sedation during the AF ablation procedure.
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The use of focal cryoablation for PV isolation and left atrial (LA) linear ablation are feasible but associated with a greater recurrence rate of reconnection.
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Focal cryoablation can be used in combination with energy sources or catheter design at specific sites to achieve safe and effective PV isolation.
Catheter-based ablation techniques are an established curative therapy for treating atrial fibrillation (AF). Currently, radiofrequency (RF) energy is the most widely accepted and used method for catheter ablation of AF. However, tissue heating with RF ablation has potential disadvantages including endocardial disruption, charring, platelet activation, pulmonary vein (PV) stenosis, and thrombus formation, and has limited efficacy in performing atrial linear ablation. As a result, new alterative energy sources have been investigated for AF ablation. These new technologies, including cryoablation, microwave, ultrasound, and laser, have different potential advantages over RF, but all of them have limited clinical experiences. This chapter discusses the current status of transvenous cryoablation for treatment of AF.
Current Status for Atrial Fibrillation Ablation
Current approaches of catheter ablation for AF are developed on the basis of the surgical maze procedures and the recognition of arrhythmogenic foci from the PVs, or less commonly from other atrial sites (superior vena cava, coronary sinus, left atrial [LA] posterior wall, vein of Marshall, and interatrial septum). These different catheter ablation techniques for AF are targeted to electrically isolate the PVs from the left atrium (segmental ostial isolation) and/or to modify the LA substrate around the PVs (wide area circumferential ablation). A recent worldwide survey shows that catheter ablation of AF can achieve a successful rate of ~80% after a mean of 1.3 procedures, and up to 70% of patients did not require further antiarrhythmic agents with different catheter ablation techniques using conventional RF ablation. However, major complications were observed in 4.5% of patients, including tamponade (1.3%), vascular access complications (0.9%), stroke and transient ischemic attack (1.1%), significant PV stenosis (0.3%), phrenic nerve injury (0.2%), and rarely, fatal atrioesophageal fistulas (0.04%). The overall risk for death after AF ablation was ~1 in 1000. The initial promise for the development of cryoablation is to minimize the energy-dependent complications of AF ablation: (1) reduce tissue fibrosis to minimize the potential of PV stenosis, (2) eliminate the risk for atrioesophageal fistulas, (3) reduce the risk for endocardial disruption/perforation to avoid tamponade, and (4) decrease thrombosis formation to reduce the likelihood of stroke and transient ischemic attack. Furthermore, cryoablation does not cause pain, which potentially reduces the need for sedation during the AF ablation procedure.
Basic Principle of Cryoablation
Cryoablation induces cellular damage mainly via disruption of membranous organelles. Although the gross architecture of the myocardium appears to be preserved after a single freezing/thawing cycle, this is followed by an initial phase of hemorrhage, microvascular thrombosis and inflammation, and then a later phase of fibrosis. As a result, cryoablation creates minimal endocardial disruption and preservation of underlying tissue architecture ( Figure 14–1 ). These properties of cryothermal lesions have the potential advantage when ablation is required within venous structures, such as PV, coronary sinus, and superior venous cava, or at LA sites close to adjacent structures, such as phrenic nerve, esophagus, and coronary artery. Indeed, animal studies have demonstrated that cryoablation did not induce stenosis of PV or coronary sinus. Furthermore, catheter cryoablation was associated with less activation of platelet and a lower thrombogenic tendency than RF energy.
Based on the principle of Joule–Thompson effect as used in surgical cryoablation probe, injection of nitrous oxide into the inner tube and then escape from the shaft into the outer lumen of a catheter can generate a temperature as low as −80°C to −90°C at the tip of the catheter ( Figure 14–2 ). Experimental studies in thigh muscle preparation have demonstrated that catheter cryoablation can create lesion size comparable with conventional 4-mm tip RF ablation. Furthermore, increasing the duration of application more than 2.5 minutes, using repeated freeze/thaw cycles at shorter cycle durations, and orienting the catheter to enhance/increase tissue contact can facilitate the creation of a larger lesion with catheter cryoablation. Nevertheless, optimal lesion creation still depends on tissue contact, as well as the local warming effect from the surrounding blood flow. As a result, the lesion of cryoablation at PVs can be limited by the high blood flow in PV. Therefore, occlusion of the blood flow in the PV during cryoablation can further increase the lesion dimension, which is one of the potential advantages of using balloon technique with cryoablation.
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